Suddenly, all files uploaded are 0 bytes -- totally baffling!

I maintain a telephone directory where each letter gets a web page. For years, I have uploaded changes to a server without incident.

I recently made a change on a couple of letters and uploaded those to files to the server. Both files appear as 0 bytes.

Figuring that maybe I did something wrong and introduces an error when I changed those two files, I decided to do a test. I uploaded another file (letter Z) without making any changes. In other words, I was uploading the exact same file that had been successfully uploaded months earlier. But now it, too, has zero bytes and can not be displayed.

I really have no idea where to start because I'm totally baffled. Anyone have an idea?